1. Field of the Invention
The present invention relates to catalytic combustors. More specifically, the invention relates to an assembly within a catalytic combustor for supporting the catalyst.
2. Description of the Related Art
Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and therefore the shaft, to rotate. The shaft may be connected to a generator.
Typically, the combustor assembly creates a working gas at a temperature between 2,500xc2x0 F. to 2,900xc2x0 F. (1371xc2x0 C. to 1593xc2x0 C.). At high temperatures, particularly above about 1,500xc2x0 C., the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO2, collectively known as NOx. The formation rate of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOX will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced.
Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature/fuel rich regions of non-well-mixed flames add to a flame""s stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in conjunction with a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creating radicals which aid in stabilizing combustion at a downstream location within the combustor assembly.
Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NOx emissions. It is, therefore, desirable to have a combustor assembly that bums a fuel lean mixture, so that NOx is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel rich mixture contains sufficient mixture strength, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame, when subjected to compressor discharge temperatures. One flow stream is mixed with fuel, as a fuel rich mixture, and passed over the catalyst bed. The other flow stream may be used to cool the catalyst bed.
One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. Prior art catalyst beds included tubes. These tubes were susceptible to vibration because they were cantilevered, being connected to a tube sheet at their upstream ends. Support for these tubes may be provided by flaring the tube ends, which has the disadvantage of thinning the walls of the tubes by as much as 25%, or providing a baffle-type tube sheet at the downstream end of the tubes, which has the disadvantage of creating counterflow at the downstream end. The inner surface of the tubes were free of the catalyst material and allowed a portion of the compressed air to pass, unreacted, through the tubes. The fuel/air mixture passed over the exterior of the tubes, and reacted with, the catalyst bed. Then, the compressed air and the fuel/air mixture were combined. The compressed air absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed.
The disadvantage of such systems is susceptibility of the tubular configuration to vibration damage resulting from: (1) flow of cooling air inside of the tubes, (2) flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and (3) other system/engine vibrations. Such vibration has caused problems in the power generation field, including degradation of the joint (e.g. braze) connecting the tubes to the tubesheet and degradation of the tubes themselves, both resulting from tube to tube and/or tube to support structure impacting.
Other proposed catalyst supporting structures include various corrugated plates, having a catalyst coating on one side. Such structures do not provide for the flexibility in gas flow patterns necessary to optimize the efficiency of the combustor.
Accordingly, there is a need for a support structure for a catalyst within a catalytic combustor having the necessary strength to withstand vibrations within the harsh environment of the combustor. Additionally, there is a need for a support structure for a catalyst within a catalytic combustor providing for multiple variations of gas flow patterns through the combustor.
The present invention is a catalyst supporting assembly for a catalytic combustor, providing increased structural support for the catalyst bearing surfaces, and increased flexibility in directing the air flow through the combuster.
The catalyst supporting structure includes a plurality of rectangular, tubular subassemblies. Each rectangular, tubular subassembly includes a pair of relatively wide surfaces hereinafter arbitrarily referred to as the top and bottom (although any orientation may be utilized). A pair of relatively narrow sides connects the top and bottom along their edges, thereby defining a tube between the top, bottom and pair of sides. The top and bottom may be slightly bent to add structural rigidity. One set of surfaces is coated with a catalyst. In one preferred embodiment, the outside surfaces of the top and bottom are coated with a catalyst. The sides, and the inside of the tube remain uncoated.
A plurality of such catalyst-coated subassemblies may be combined, with the top surface of one subassembly facing the bottom surface of an adjacent subassembly. A pair of support walls on either side of the subassemblies may include channels dimensioned and configured to receive the sides of the subassemblies, thereby supporting the subassemblies along their entire length. The resulting catalytic combustor includes a plurality of alternating catalyst coated and uncoated channels, with the uncoated channels preferably defined within each subassembly, and the coated channels preferably defined between adjacent subassemblies and the support walls.
The number of channels within a catalytic combustor may vary within a wide range, permitting a great deal of flexibility in selecting this number to meet a particular need. For example, a small number of subassemblies may fit within a rectangular catalytic combustor, or a larger number of small size subassemblies may fit within a trapezoidal combustor assembly section. A plurality of trapezoidal subassembly sections, for example, six sections, may surround the pilot nozzle of a combustor.
The flow of cooling air and fuel-air mixture through such a catalytic combustor assembly may be configured along several possible paths. For example, one preferred path may direct cooling air into the subassemblies from one end of the combustor, through the subassemblies, and out the other end of the subassemblies. Within the same embodiments, another portion of the air may be mixed with a fuel, and then directed through openings in the support walls into the catalyst coated channels, wherein it turns downstream and flows through the catalyst coated channels. The cooling air and fuel-air mixture then mix at the end of the combustor assembly.
An alternative preferred cooling air and fuel-air mixture path begins by directing air flowing adjacent to the catalytic combustor through a manifold along the side of the series of alternating coated and uncoated channels. The air may then be passed through openings in the sides of the subassemblies, traveling through the subassemblies and substantially perpendicular to these subassemblies, exiting through openings in the other side of the subassemblies and into another manifold. The cooling air has thereby absorbed some heat from the catalyst-coated channels adjacent to the uncoated channels through which the cooling air has passed. The cooling air may then be directed towards the intake end of the coated channels, and then directed through these channels.
The advantage of such a flow path is increased efficiency from preheating the fuel-air mixture by first utilizing the air to absorb heat from the catalyst coated channels. The preheated air is then mixed with fuel, and passed through the catalyst-coated channels, wherein the reaction between the fuel air mixture and the catalyst causes the fuel-air mixture to heat further.
It is therefore an aspect of the present invention to provide a catalyst supporting structure for a catalytic combustor, wherein the catalyst is applied to rectangular, tubular structures.
It is another aspect of the present invention to provide a catalyst supporting structure for a catalytic combustor, wherein the structure has increased structural support, and resistance to vibration.
It is a further aspect of the present invention to provide a catalytic combustor having multiple possible paths for the cooling air and fuel-air mixture.
It is another aspect of the present invention to provide a catalyst support structure for a catalytic combustor wherein the catalyst is applied to a large surface area, thereby reducing the overall number of parts.
It is a further aspect of the present invention to provide a catalyst support structure for use in catalytic combustors, wherein the various subassemblies may be maintained the proper distance apart, within tight tolerances, without the need for shimming.
It is another aspect of the present invention to provide a catalyst support structure for catalytic combustors wherein the size of the various subassemblies may be easily varied to accommodate various combustor envelopes.
These and other aspects of the invention will become apparent through the following description and drawings.